The Thomas Fire — one of the largest wildfires in California history — was sparked by Southern California Edison power lines that came into contact during high winds, investigators said Wednesday.

The resulting arc ignited dry brush on Dec. 4, 2017, starting the blaze in Ventura and Santa Barbara counties that resulted in two deaths and blackened more than 440 square miles, according to the investigation headed by the Ventura County Fire Department.

The arc "deposited hot, burning or molten material onto the ground, in a receptive fuel bed, causing the fire," said a statement accompanying the investigative report.

Southern California Edison didn't immediately return a call seeking comment.

The fire destroyed more than 1,000 structures before it was contained 40 days after it began near the city of Santa Paula. A firefighter and a civilian were killed.

A month after the blaze started, a downpour on the burn scar unleashed a massive debris flow that killed 21 people and destroyed or damaged hundreds of homes in the seaside community of Montecito. Two people have not been found.

The investigation was conducted by fire officials in both counties along with Cal Fire.

Investigators said the Thomas Fire first began as two separate blazes that joined together. They determined the utility was responsible for both ignitions.

Edison acknowledged last fall that its equipment likely started one of the two fires.

Victims claimed in lawsuits that losses from the blaze and flooding were due to negligence by Edison, which has said it will work with insurance companies to handle the claims.

The utility is protected from going bankrupt over the disasters, thanks to a law signed last year that passes excess liability costs on to utility customers.